Reflective telescopes are commonly used in astronomy as well as in various other applications for imaging remote objects at high magnification. The reflective telescopes often include rear or front refractive optical elements and in these cases are referred to as catadioptric telescopes.
Reflective telescopes are often designed such that an inner telescope space between the reflectors (e.g. mirrors) is encompassed by a tubular structure. The telescopes are often subjected to external and internal conditions causing differences of temperature between the primary mirror, secondary mirror and tubular structure. Air filling the inner telescope space resides in heat transfer with the mirrors and the structure. As a result, temperature gradients and convection currents are created in the air. The phenomenon is often referred to as “telescope tube currents” and sometimes as “thermal wake”. The temperature gradients cause inhomogeneity of the air refraction index inside the telescope's optical path, resulting in optical wavefront error. This optical wavefront error degrades the optical resolving power of the telescope and represents a major issue to be addressed (Mobberley M., Lunar and Planetary Webcam User's Guide, pp. 33-36, Springer, 2006; Sidgwick, J., Gamble R., Amateur Astronomer's Handbook, pp. 200-202, Courier, 1971).
In some reflective telescopes such as in Newtonian telescopes having a rear primary mirror with no central opening, fans are used to reduce the temperature gradients. In some solutions, these fans are positioned behind the primary mirror, creating forced heat transfer of the latter with ambient air, and equalizing its temperature with one of the ambient air. In other solutions, the fans are positioned at side openings of the telescope housing, in vicinity of the primary mirror's optical surface. In this case the fan operation not only equalizes the mirror temperature with one of the ambient air, but also directly prevents development of convection air currents in vicinity of the mirror's optical surface (http://www.skyandtelescope.com/astronomy-equipment/beating-the-seeing/).
Other telescope configurations such as Cassegrain or Schmidt-Cassegrain telescopes have a rear primary mirror with a central opening and a front secondary mirror. In the Schmidt-Cassegrain telescopes the front end of the telescope housing is typically sealed by a glass plate used, inter alia, for correcting optical aberrations. The rear primary mirror although having a central opening perforation is typically sealed or semi-sealed at a rear side thereof, as it usually interfaces an optical detector or optical element for directing the incoming light therethrough. This configuration prevents ventilation of the telescope inner space and, under transient or uneven temperature conditions, suffers from severe optical sharpness loss due to the air refractive index inhomogeneity.
When using a reflective telescope such as the Cassegrain in an electro-optical payload, the telescope often operates in transient temperature conditions, arising for example, from altitude change. In the transient conditions, components of the telescope (primary and secondary mirrors, tubular structure) cool down or heat up at different rates, therefore creating considerable temperature gradients in the telescope's internal volume.
Moreover, even at steady-state conditions, the telescope is typically subjected to inhomogeneous thermal boundary environment. Inside the payload, typically there are numerous heat sources such as electronic boards, cryo-coolers, motors and focal plane arrays. Heat fluxes from these sources enter the telescope's tubular structure and primary mirror back, causing uneven heating. However, the telescope's front end and secondary mirror are typically significantly cooler, being located in vicinity of an optical window subjected to an ambient temperature at its external side.As a result of the effects, significant air temperature gradients develop inside the telescope volume, and optical sharpness is strongly degraded because of the air refraction index inhomogeneity.
U.S. Pat. No. 3,791,713 discloses a dynamic reflective telescope system having a support and a hollow spherical housing rotatably mounted thereon. A telescope is fixedly mounted within the hollow spherical housing for rotational movement therewith. The telescope is thermally insulated from the interior of the hollow spherical housing thereby defining two distinct volumes, the volume within the telescope being maintained at essentially the same temperature as the surrounding atmosphere, whereby definition is improved by eliminating variations in the index of refraction within the telescope. Monitoring means adjusts the tangential and radial position of the individual primary mirror portions to compensate for atmospheric turbulence.
Patent application No. CN106772999 relates to the field of astronomical telescopes and discloses a lightweight primary mirror back heat control system of a large-aperture telescope, aiming to equalize the primary mirror temperature with the ambient temperature, as well as to facilitate the rejection of the mirror heat into the ambient environment in a way minimizing the telescope vibration which could be caused by the fan operation. The lightweight primary mirror back heat control system comprises a heat control fan device and a plurality of air nozzles, and the heat control fan device comprises an axial fan, a heat exchanger, eight damping rubber cylinders, four mounting angle plates, two reinforcing plates, four fixing suspension rods, four suspension rod transition plates, a flexible corrugated pipe, a mirror chamber air duct transition pipe and a heat exchanger air duct transition pipe. The lightweight primary mirror back heat control system of the large-aperture telescope has the advantages that the system is capable of providing the cold air circulation while controlling the cold air temperature to facilitate the heat transfer, and cooling fluid in a heat exchanger can bring heat of a primary mirror of the telescope away, while preventing vibration of the axial fan from transferring to the primary mirror.
U.S. Pat. No. 7,471,451 discloses a multiple field of view optical system of a type applicable in stabilized electro-optical payloads. The optical system includes a reflective telescope serving as a magnifying stage for its narrow field-of-view. The central opening in the telescope's primary mirror is occupied by lenses and therefore closed for passage of air. Hence no ventilation can be provided to the telescope's internal volume, and the system is prone to optical sharpness deterioration because of air temperature gradients.
U.S. Pat. No. 8,848,290 discloses a method of control and mitigation of thermal wake proximate to an optical element of a telescope. The position and angular orientation of a secondary mirror is finely adjusted by controlled heating of supporting struts. To prevent creation of the thermal wake in the adjacent air because of the strut heating, each strut is covered with a shroud, with a fan facilitating airflow inside the shroud, and the extracted hot air is directed outwards, not entering the telescope's optical path.
Japanese patent JPH6250102 discloses a system for rejection of heat created by control mechanisms located behind a primary mirror of an astronomical telescope. As a result, the primary mirror is maintained at temperature close to one of the ambient air; thus the convection currents and refraction index non-uniformity are prevented. The system includes a hollow enclosure located behind the primary mirror and encompassing the heat-dissipating control elements, and fans forcing air flow through the enclosure. The enclosure represents a heat exchanger, and is not a part of the optical path.